Whenever I get emails from the VAF forums (for PMs, subscribed threads, etc), Gmail flags them as (unknown sender) and marks them as spam. Google's reason: "It is in violation of Google's recommended email sender guidelines. "

A snippet from an email header of a VAF email gives some clues:

Code:

From: "VAF Forums" <>
Return-Path:

...and then further down..

Code:

Return-Path: <>

Maybe the blank From address or blank return path have something to do with it? I googled "vbulletin messages marked as spam" and found this

You may want to try to change the webmaster email to an email address from your domain name. You should also try the -f option and if that does not work, you can request that your host check their mail settings/error logs or contact gmail to determine why you are being flagged as spam.